Y is formed when X is heated in the absence of air. Y is a tough, porous, and black substance. Both X and Y are carbon-rich materials. What could X and Y be?

When coal is heated in the absence of air, it undergoes a process called destructive distillation, which produces coke. 

Coke is a tough, porous, and black substance that is used in the manufacturing of steel and other industrial processes. Both coal and coke are carbon-rich materials.